Time Reversal Symmetry Breaking in Topological Insulator/Magnetic Insulator Heterostructures Revealed by the Negative Magnetoresistance

Topological insulator (TI) is one of emergent quantum materials exhibiting topological surface states (TSSs) with spin momentum locking. Breaking time reversal symmetry (TRS) of TSSs leads to novel phenomena such as quantum anomalous Hall effect (QAHE). In this work, we utilized magnetic insulator yttrium iron garnet (YIG) to break TRS through the magnetic proximity effect (MPE) that enables uniform magnetization without introducing crystal defects, as opposed to magnetic dopings of Cr into TI. High quality YIG thin films were obtained by sputtering followed by high temperature anneals, and Bi₂Se₃ thin films were deposited at 280 ^○C by MBE. Carrier concentrations and mobilities of Bi₂Se₃/YIG were found to be comparable to those of Bi₂Se₃/Sapphire. It was observed that, in contrast to Bi₂Se₃/Sapphire, the distinctive negative magnetoresistance showed in Bi₂Se₃/YIG as the temperature decreased. Magnetoconductances can be well described by incorporating a weak localization term into the fitting function, which indicates the gap opening of TSSs according to the theory developed by Hai-Zhou et al. The systematic dependence of the WL and WAL components on Bi₂Se₃ thickness varying from 7 to 40 nm will be reported. Our study may pave the way to realize QAHE at higher temperatures.